I had a 80g external hard drive that worked fine on my mac but the cord finally gave out. so I removed the hard drive, in order to install it on my mac as a 2nd backup drive. But when i turn the mac on it shows a blank screen with the finder folder blinking in the mid. screen. what can i do?!!!

I;m not sure because i don't really know, below is what the seagate (the external hard drive's maker) website says.

How do I know if the jumpers are configured correctly?
The correct jumper settings are very important to a successful drive installation. Jumper settings for all Seagate hard drives are available in both graphical and text-only formats. Additionally, all new Seagate hard drives now have the jumper settings printed on the outer casing of the drive.

Most Macs provide termination power for the SCSI bus, making the most common Term Power setting for Seagate Hard drives "From the Bus." The Mac Plus, Portable, 100-series PowerBooks, 500-series PowerBooks, the PowerBook 1400, and Macs using ATA (IDE) drives do not supply SCSI termination power. For more information on which models do and do not provide SCSI termination, visit Apple's AppleCare Knowledge Base.

Most Mac OS systems that support ATA drives support only one drive, the Master. Make sure the drive is jumpered for Single/Master.

Some newer Mac OS systems support two ATA drives, Master and Slave. Make sure that both drives are jumpered properly for their respective positions on the bus.

If the drive originally came with Mac OS system, please contact Apple (or your Mac OS computer maker) for information about the drive.

First OFF with SCSI drives there is no Master or Slave as with IDE drives. But you do need to set the ID number for the SCSI chain. You said that you pulled the drive from an external case. Seagate offers two basic configuration one model end in LC the other LW the difference is the LC model is designed to mount in an external case.
Go to Seagate under support locate your drive model you may have to register to get the jumper information. Once you find your drive look at the jumper setting for setting the SCSI ID #. The internal boot drive is set to 0 and the host adapter is set to 7 so pick 2 thur 6. If you have only one jumper pin then pick an ID that only needs a single jumper. That should do it. Install and start ur up.